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Today I have seen this report of a SpaceX Falcon 9 rocket test firing. The force of a burning rocket is tremendous, and in the video we see an apparently simple construction keeping the rocket (or rocket stage in this case) tied to the ground.

How does this construction manage to keep the rocket on the ground? What are its materials and design? Has this same (or similar) design always been used for such tests?

Falcon 9 rocket firing test

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    $\begingroup$ I think those thin wires from the top of the rocket stage down to the ground are only used to keep to rocket upright, but there are some heavy bolts between the bottom of the stage and ground not visible in this picture. The bolts will hold the tremendous force. $\endgroup$ – Uwe Aug 1 '16 at 14:01
  • $\begingroup$ I don't have enough fact to justify an actual answer, but some conjecture from various space communities I've visited -- The usual pad support clamps handle the initial thrust. As the tanks near zero some additional support is needed to fight the growing TWR. It's also guessed that manually tensioning the cables could simulate some of the aerodynamic forces experienced in flight, putting load on the airframe. $\endgroup$ – Saiboogu Aug 4 '16 at 15:06
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I can answer this part "Has this same (or similar) design always been used for such tests?"

In the early days of the program Shuttle would occasionally do a pad test where the main engines were fired briefly.

During this test (and at all other times) the Shuttle/External Tank/Solid Rocket Booster stack was held down to the Mobile Launcher by eight bolts. Four of these bolts connected the aft skirt of each Solid Rocket Booster to the Mobile Launcher. The bolts were 28 inches long and 3.5 inches in diameter. You can see the hold-down post which was the interface between the bolts and the Launcher in the picture below.

enter image description here

A schematic of the holddown post and bolt is shown here.

enter image description here

For a real launch, the nuts were blown in half by pyrotechnic devices (2 per nut, for redundancy). Here is a picture of the nuts after launch.

enter image description here

I am not sure you can accurately characterize this as "simple" but it was generally reliable. I am aware of a couple of problems that occurred throughout the years, the amusingly named "stud hang ups" and an incident on STS-112 where one set of redundant pyros didn't fire.

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    $\begingroup$ It's worth noting that the SSMEs themselves couldn't lift the shuttle -- they only lift about a quarter of the fully-fueled weight. You'd still want the hold-downs engaged, obviously, but... $\endgroup$ – Russell Borogove Aug 2 '16 at 2:48
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    $\begingroup$ Yes, due to the off angle thrust they flexed the SRBs enough to move the crew cabin a couple of feet (the "twang")....the stack would have toppled over if not restrained. youtube.com/watch?v=ExfjSuJxOP8 $\endgroup$ – Organic Marble Aug 2 '16 at 3:27
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Lets assume 5*10^6 lbs of thrust:

Then 4 steel wires of approximately 60mm diameter should be enough.

The special thing about a rocket is not it's maximum force at zero speed but the fact that the force is generated in a way which is independent from the velocity difference to a medium in which the vehicle flies, and stays constant.

Source for the strength of the steel wire: DIN3069 wire (DIN: Deutsche Industrie Norm, usualy equivalent to some ISO norm) http://drahtseiltechnik.cgahrens.de/edelstahlseile/bruchlastrechner

Breaking load table (starting from Page 21/22) on: http://www.wirerope.com/docs/ctg-industrial.pdf

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  • $\begingroup$ I like your explanation, but can you add some kind of supporting information? What strength did you use for the steel? Do you really mean "wire" as in flexible, or maybe bolts instead? It improves the answer by helping other readers learn from the information as well. Double check for a decimal point off by one place $\endgroup$ – uhoh Jul 31 '16 at 11:38
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    $\begingroup$ added the sources. $\endgroup$ – Sascha Jul 31 '16 at 13:16
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    $\begingroup$ Excellent! High tensile steel is amazing stuff! I'd just used normal structural steel - the factor of 10 comes from my choice of material. Thanks for the information! (I'd still rather hold it down with bolts, but that's just me.) $\endgroup$ – uhoh Jul 31 '16 at 13:27
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    $\begingroup$ I thought the Falcon 9 had more like 1.5 million pounds of thrust... $\endgroup$ – 2012rcampion Jul 31 '16 at 19:42
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The launch mount used for actual SpaceX launches is capable of holding the full power of the stack for at least 1 second before every launch, and for the hot fire tests that are often several seconds long.

The SpaceX ones are in some ways simpler (no explosives) but in some ways more complex (they release on demand) than the Space Shuttle ones.

The stack in the image above is the new test pad at McGregor. They built it, we think to test a Falcon Heavy at full thrust, which is almost 5 million lbs of thrust.

They used a LOT of concrete, dug down deep into the ground, with lots and lots of dirt/weight on top of it, so that the rocket does not launch with the launch mount attached.

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    $\begingroup$ Thanks for putting the image of an accidental liftoff with the test pad attached to the base of the rocket in my head. $\endgroup$ – Jörg W Mittag Aug 1 '16 at 3:42
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The orange top and strong cables seem to be a bit atypical. There is a video of F9-019 (Jason-3) on the same stand. which shows thinner cables and no special cap. But it is a v1.1 of F9 so some differences may be because of that too.

The most important part of the hold-down mechanism are the clamps which keep the rocket fixed to the transporter/erector or a test stand. These attach directly to the octoweb structure just above the engine bells. There is a shot of the release in SpaceX webcast intro (you can see it for example at 7s into Thaicom 8 webcast from which I took this image)

enter image description here

This seems to be the old F9 v1.0 if I am not mistaken. Later in the intro there is another detail of the release at 14s. Then in the same webcast there is a closeup on the current version at 21:20 but unfortunatelly without the actual release.

enter image description here

About the orange cap and thick wires - the speculations are that it is used to simulate the weight and some stressed (from vibrations etc.) of the second stage to check the structural integrity of the returned stage.

(Used images belong to SpaceX)

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